No offense, but I really don't think that this is going to work, man. It just seems like wasting money =(
1. Aro is already in the tank before introducing discus.
2. 2-3 Discus in a tank with aro...they won't feel safe (or be safe). You need minimum 6 (preferably 10+) full grown adults (minimum 6-7 inches each or they'll get eaten eventually). If you get juveniles or subadults you need to raise them in a separate tank until they are full grown...unless you want to 1) risk them being eaten by the aro or 2) change 40% of the water daily in the large aro + discus tank as the young discus need to be fed 4-6 times a day for full growth (which you will need if housed with aro).
3. You can buy adults but disease free, healthy adult discus run minimum $100-150 each
4. You would have to QT the aro to make sure it doesn't have anything that will infect the discus.
5. You will need a giant 500+ gallon tank to adequately house silver aro + discus + rays. The aro gets huge. Rays need lots of bottom area. Discus need large room to feel safe from large, steadily moving fish like aros.
Hope this helps man.